stackable-box: Only count allow-visible=true children for index

Fixes Librem5/libhandy#213

Signed-off-by: Alexander Mikhaylenko <alexm@gnome.org>
11 jobs for wip/exalm/separators in 15 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #250616
librem5
api-visibility

00:02:49

passed #250618
librem5
build-debian-clang

00:03:53

passed #250617
librem5
build-debian-gcc

00:03:07

passed #250619
librem5
build-fedora-gcc

00:04:35

 
  Test+Docs
passed #250620
librem5
build-gtkdoc

00:02:23

passed #250621
librem5
unit-test

00:02:20

41.9%
 
  Package
passed #250622
librem5
package-debian-buster

00:02:28

passed #250623
librem5:arm64
package-debian-buster:arm64

00:05:49

passed #250624
flatpak
package-flatpak

00:01:41

 
  Test Package
passed #250625
librem5
autopkgtest-debian-buster-package

00:02:21

passed #250626
librem5
lintian-debian-buster-package

00:00:55